The invention relates to a screw drive with rolling elements and, more particularly, to a ball screw drive including a screw spindle with a spindle axis and a double nut running on the screw spindle with two separately formed single nuts which are substantially rigidly joined in the direction of the axis of the spindle. In order to set a desired play or bias relative to the screw spindle, the two single nuts can be turned continuously about the axis of the spindle relative to each other and fixed in any relative rotary position corresponding to the desired play or bias.
Screw drives of the foregoing type are used, for example, for rapid and accurate positioning of tool and work holders and for moving them at defined speeds. Rotary motion of the screw spindle is converted to linear motion of the guided double nut. In order to satisfy the highest possible demands for accuracy in reversing the direction of motion of the double nut, the double nut has to be arranged on the spindle as far as possible without play. For this purpose, when the two nuts are being mounted on the spindle they are turned towards each other far enough to make their facing end surfaces bear against each other. Further turning of the nuts causes the rolling elements to be applied to opposing side surfaces of the raceways of the nuts, which ultimately results in the desired biasing of the two nuts relative to the screw spindle and guidance of the double nut on the spindle substantially without play.
A screw drive of the above type, i.e., a screw drive in which the play or bias of the two single nuts relative to the screw spindle can be infinitely (steplessly) adjusted, is known, for example, from DE-OS 21 35 812. A screw bolt is contained in an intermediate element of the known screw drive which is non-rotatably joined to one of the single nuts, the screw bolt being in screw engagement with a part mounted non-rotatably on the other single nut. The worm drive thus formed allows infinitely variable turning of the two single nuts relative to each other about the axis of the spindle. One disadvantage of this construction is that a relatively large amount of space has to be provided for the worm drive, particularly in a radial direction. Another disadvantage is that, in spite of the self-locking inherent in worm drives, the desired bias set may undesirably re-adjust itself, for example, due to vibrations, causing the screw bolt of the worm drive to turn.
A similarly constructed screw drive is known from DE-OS 23 49 958.
Versions of the generic screw drive are known from DE-OS 23 49 958 and from DE 30 38 774 C2 and the corresponding EP 0 049 903 B1 in which an intermediate element is in frictional engagement with both single nuts in order to prevent relative rotation between them. The frictional forces must therefore be overcome during the assembly, i.e., the relative turning of the two single nuts to set the desired bias or play. This makes exact setting of the play or bias to the desired value considerably more difficult.
Screw drives are further known from DE 24 53 635 C3, DE-OS 25 33 996 and DE 29 40 762 C2 in which the two single nuts can only be turned relative to each other and fixed in predetermined angular steps rather than infinitely variably.
In the screw drive known from German Utility Model 77 08 184, infinitely variable turning of the two single nuts is only possible within a very narrow angular range. In order to set a desired bias or play relative to the screw spindle, it is therefore also necessary to provide spacers between the two nuts, and their thickness must be determined by the desired bias or play. Hence, even with experienced operators it is usually necessary to change the spacers several times during assembly, until the spacer with the right thickness for the desired bias or play has been found. Assembly of the screw drive known from DE-GM 77 08 184 is therefore laborious and time consuming.
In DE 39 00 693 C2, a cavity formed by annular grooves and indentations between the two touching end faces of the single nuts is filled with casting compound to form the intermediate element. The position of relative rotation of the two single nuts corresponding to the desired bias or play must therefore be maintained until the casting compound has completely hardened.
A screw drive where the two single nuts are joined by an intermediate element which yields resiliently in the direction of the axis of the spindle is known from DE 42 08 126 A1. An axially resilient arrangement of this type has the drawback that, when a force exceeding the spring force is exerted on one of the single nuts, the two single nuts may move towards each other, thereby losing the desired bias.
In DE 32 07 566 A1, after the desired bias of the two single nuts has been set, a hole to receive a radial locking bolt is formed in their common contact region, to ensure that the bias obtained is maintained.
An object of the invention, therefore, is to provide a rolling-body screw drive of the above-mentioned type which, with a straightforward construction and straightforward assembly sequence, readily enables the play or bias of the single nuts relative to the screw spindle to be adjusted and set in a stepless and precise manner to a desired value.
According to the invention, the foregoing object is attained by the provision of a screw drive of the above type, in which either a first of the two single nuts or an intermediate element joined thereto and arranged between the two single nuts has at least one peripheral section pressed into an associated aperture in the second single nut. In accordance with the invention, the non-rotational joining of the first single nut or the intermediate element to the second single nut is the last operation in assembling the screw drive according to the invention. The possibly necessary non-rotational joining of the intermediate element to the first single nut or the formation of the intermediate element on the first single nut may take place in a preceding operation or simultaneously with the non-rotational joining of the intermediate element to the second single nut.
The limit between direct joining of the two single nuts and joining an intermediate element integral with one of them to the other single nut is a fluid transition in practice. It is not always possible to decide which of the two extreme cases applies. Basically any part of the single nut used for joining the other single nut may be considered as an intermediate element, even when that intermediate element is not a separately formed part of the single nut in question.
In order to join the two single nuts or the intermediate element non-rotatably to the second single nut, the latter is first brought into contact with the first single nut or the intermediate element by turning on the screw spindle. The desired play or bias of the two single nuts relative to the screw spindle is then set by further turning of the nuts. The only frictional forces arising between the intermediate element and the nut emanate from the actual bias. When the play or bias has reached the desired value, a peripheral section of the first single nut or of the intermediate element is then pressed into an associated aperture in the second single nut. A secure, non-rotational joint is thus obtained, allowing immediate further processing of the screw drive. Subsequent accidental turning of the two single nuts relative to each other, with resultant re-adjustment of the desired play or bias, is reliably avoided.
In accordance with the invention, the peripheral section may be pressed into the aperture in the second single nut in an axial or/and radial direction. It is then advantageous for the peripheral section which is pressed in to bear on the wall surfaces bounding the aperture only in a peripheral direction. In this way, one can reliably avoid forces acting in either an axial or a radial direction that might influence the bias or play of the two single nuts relative to the screw spindle.
In order to reduce the impressing forces required or/and to enable the peripheral section pressed in to be applied to the side walls of the aperture over the largest possible area, provision may further be made for the peripheral section which is pressed in to be indented.
To increase the stability of the whole arrangement and thus reduce the risk of deformation of the nuts, particularly in their end regions facing towards each other, the aperture may be bounded radially inwardly by a wall section of the second single nut.
In certain applications it may be advantageous for the first single nut or the intermediate element to be additionally adhered to the second single nut, preferably using a double-sided adhesive tape. Adhesive tapes with curing synthetic resins suitable for this application are obtainable, for example, under the name of SCOTCH(trademark) VHB(trademark).
Also, the second single nut may clearly be joined to the first single nut or the intermediate element by means of a plurality of sections of the first single nut or intermediate element pressed into apertures in the second single nut spread over the whole periphery.
There are various possible ways of joining the intermediate element to the first single nut. Thus, the intermediate element may both be integral with the first single nut and an element separate from that nut. In the first case, a particularly stable non-rotational connection is obtained between the intermediate element and the first single nut, although it makes it difficult to dismantle the double nut of the screw drive on the screw spindle without breaking it.
For example, the intermediate element may be joined to the first single nut with a press fit. Additionally or alternatively, the intermediate element may be adhered to the first single nut, preferably using a double-sided adhesive tape, as already mentioned for joining the intermediate element to the second single nut.
To obtain a positive connection between the intermediate element and the first single nut, the intermediate element may be held non-rotatably relative to the first single nut by at least one additional fit component, which is joined to or integral with one of the two components, i.e., the intermediate element or the first single nut, and which in either case engages in an aperture provided in the other component, the first single nut or the intermediate element.
In a first alternative form of the positive connection, the additional fit component may be formed by a peripheral section of the intermediate element which is pressed into an aperture in the first single nut. In a second alternative form, it is also possible for the additional fit component to be a separate component and to be fixed detachably to the first single nut, for example, by being screwed onto it by means of a screw bolt. The advantages of these two alternative joining methods may be seen from the above discussion of the various ways of joining the intermediate element to the second single nut. In particular, if an additional fit component which can be screwed onto the first single nut is used, it is then possible for the inside diameter of the hole passing through the additional fit component to be larger than the outside diameter of the screw bolt, and for the hole to be provided with an internal screw thread.
Another possible way of joining the first single nut to the intermediate element is to have a plurality of additional fit components distributed around the whole periphery.
Another method of joining the intermediate element to the first single nut is to screw it onto or into the nut, for example, by means of a fine thread. The decisive factor in making the joint between the intermediate element and the first single nut non-rotational in operation is that the pitch and/or rotary direction of the screw connection between them should be different from those of the screw spindle.
To facilitate reproducible and accurate assembly of the screw drive according to the invention it is proposed, in a further embodiment, that the intermediate element on at least one of the single nuts, and preferably both single nuts, be arranged centered in respect of the axis of the spindle, or that the two single nuts be arranged centered relative to each other in respect of the axis of the spindle.
To increase the stability of the screw drive but also to ensure high-precision manufacture, it is further proposed that the intermediate element should be formed by a closed ring member.
Further according to the invention, at least one of two components to be joined, i.e., the first single nut or intermediate element, or the second single nut or intermediate element, or one of the two single nuts, may be made of soft, i.e., non-hardened, steel at least in a region determined for joining to the respective other component. These soft regions may also be treated in a simple manner by the end user. For example, tapped holes may be made in these soft regions with ordinary commercial tools, allowing highly flexible installation of screw drives according to the invention in higher-level structures. Moreover, the soft regions of one component may be pressed into associated apertures in the other, to join the two components.
The use of case-hardened single nuts is nevertheless not excluded. The provision of at least one case-hardened single nut may be appropriate, e.g., when the intermediate element is integral with one of the two single nuts and the nut is to be joined to the other single nut by pressing in. In a case-hardened single nut for this purpose, the region to be pressed in is not carburised. Apart from case-hardening, the spindle raceways of the single nuts may also be induction hardened. This method, in which the steel to be hardened is merely induction heated instead of having carbon supplied to it, is particularly suitable for use in single nuts made entirely of carbon-rich steel. Owing to the high carbon content of the outer surface region surrounding the spindle raceway, induction hardened single nuts cannot be welded directly together. However, the outer surface region remains soft in the induction hardening process, so the single nuts can be joined by the pressing-in method described above.
In an advantageous embodiment of the invention, the intermediate element comprises an annular disk, which is connected to the first of the two individual nuts and which is arranged between the two individual nuts and has its disk plane running essentially orthogonally to the spindle axis. At least one circumferential section of the disk is pressed into an associated recess of the second individual nut. This solution has the further advantage that it only requires a small amount of space, even in the axial direction, for arranging the annular disk between the two individual nuts.
During the assembly of the rolling-body screw drive according to this embodiment, the rotationally fixed connection of the annular disk to the second individual nut may, according to a first method variant, be the last operation. For this purpose, the second individual nut is first of all brought into abutment, by rotation on the threaded spindle, against the annular disk, which is already connected to the first individual nut in a rotationally fixed manner. Further rotation of the two individual nuts then sets the desired level of play or the desired prestressing of the two individual nuts in relation to the threaded spindles. The only frictional forces occurring here between the annular disk and the nut stem from the prestressing itself. If the level of play or the prestressing has reached the desired value, at least one circumferential section of the annular disk is pressed into an associated recess of the second individual nut. This achieves a secure rotationally fixed connection which allows immediate further processing of the rolling-body screw drive. In addition, subsequent unintentional rotation of the two individual nuts relative to one another, with resulting adjustment of the desired level of play or of the desired prestressing, is reliably ruled out.
In order to simplify the assembly steps necessary for assembling the rolling-body screw drive according to this embodiment, it is also possible for the rotationally fixed connection of the annular disk to the first individual nut to be carried out by virtue of at least one circumferential section of the annular disk being pressed into an associated recess of the first individual nut. According to the first method alternative mentioned above, it is possible here, before the at least one circumferential section is pressed in, for the annular disk to be centered relative to the first individual nut using a centering pin.
It is basically also possible however, according to a second method alternative, for the annular disk to be connected to the two individual nuts in a rotationally fixed manner at the same time, advantageously in both cases by virtue of at least one circumferential section of the annular disk being pressed into associated recesses of the two individual nuts. In this case, in order to simplify the assembly, it is conceivable for a centering extension for centering the annular disk to be provided on at least one of the individual nuts.
Irrespective of the method alternative which is used for assembly in each case, centering of the annular disk in relation to the first individual nut or in relation to both individual nuts has, quite generally, the advantage of facilitating the reproducible and precise assembly of the rolling-body screw drive according to the invention.
At most, during the assembly of the rolling-body screw drive according to the annular disk embodiment, there may be a problem if, when the desired prestressing or the desired level of play of the two individual nuts has been reached, two recesses of said individual nuts are located precisely opposite one another in the direction of the spindle axis. This is because the annular disk can only be connected in a form-fitting manner to one of the two individual nuts at a certain circumferential section. In order for it to be possible to remedy this, it is proposed that, when each of the individual nuts is connected to the annular disk at a predetermined number of circumferential sections, at least one of the individual nuts, preferably the second individual nut, has a larger number, preferably at least double the number, of recesses than corresponds to the predetermined number. In addition or as an alternative, it may nevertheless also be provided that, when an individual nut is provided with at least two recesses suitable for connection to the annular disk, such recesses are distributed irregularly over the circumference of the individual nut. Finally, this problem can also be addressed by providing annular disks of at least two different thicknesses and, when it is established in relation to one disk thickness that the recesses of the two individual nuts are located opposite one another, the disk thickness is exchanged for an annular disk of a different thickness. It goes without saying the other disk thickness has to be selected such that, with this disk thickness, the problem of recesses being located opposite one another does not arise.
The annular disk preferably consists of soft, i.e., non-hardened, steel and can thus be produced, for example, by punching from sheet steel which can be obtained cost-effectively. This has the advantage that the annular disk, as an inexpensive series-production part, can be destroyed without undue concern during the dismantling of the rolling-body screw drive and can be replaced by a new annular disk during the renewed assembly of the two individual nuts.
The two individual nuts may be, for example, case-hardened individual nuts, i.e., individual nuts of which the rolling-body running surface is hardened by carburizingxe2x80x94and thus has a high carbon contentxe2x80x94and is enclosed by an outer casing region made of a softer steel with a low carbon content. Such case-hardened individual nuts have the advantage that the soft casing region can also be easily worked by end users. For example, threaded bores can be made in these soft casing regions even with commercially available tools, which allows high-flexibility installation of the rolling-body screw drives according to the invention in primary structures. Moreover, the soft regions can be pressed in to associated recesses of the primary structure. If at least the soft casing region is additionally produced from a weldable steel, that is to say, for example, from a steel with a low carbon content, then the rolling-body screw drive according to the invention may additionally be connected to the primary structure by welding. It is also basically possible, however, for the individual nuts to be induction-hardened. This hardening method, in which the steel which is to be hardened, rather being fed any carbon, is merely inductively heated, is suitable, in particular, for use with individual nuts which consist overall of high-carbon steel.
In a development of the invention, at least one circumferential section pressed into an associated recess is supported solely in the circumferential direction on that wall surface of the individual nut which bounds the recess. This reliably avoids forces which act both in the axial direction and in the radial direction and could influence the prestressing or the level of play of the two individual nuts in relation to the threaded spindle.
In order to reduce the forces necessary for the pressing-in operation and/or allow abutment of the pressed-in circumferential section against the largest possible surface area of the side walls of the recess, it may also be provided that the pressed-in circumferential section is indented.
In order to increase the stability of the overall arrangement, and thus to reduce the risk of deformation of the nuts, in particularly in the mutually facing end regions of the same, it may be provided that the at least one recess is bounded in the radially inward direction by a wall section of the associated individual nut.
In certain applications, it may be advantageous if the annular disk is additionally adhesively bonded to at least one of the two individual nuts, preferably using double-sided adhesive tape. Adhesive tapes with setting synthetic resins which are suitable for this application can be obtained, for example, under the name SCOTCH(trademark) VHB(trademark).
For as straightforward a construction as possible of the rolling-body screw drive according to the invention, it is also proposed that the two individual nuts be supported on one another in the direction of the spindle axis by means of the annular disk. It is basically also conceivable, however, for the two individual nuts to be supported directly on one another.
The invention further concerns a method of assembling a screw drive according to the invention, particularly a ball screw drive, wherein, in order to set a desired play or bias, the two single nuts are placed against each other by turning them on the screw spindle and are joined non-rotatably in a relative rotary position corresponding to the desired play or bias and wherein, in order to fix the relative rotary position of the two single nuts, the first of the two single nuts or an intermediate element joined thereto and arranged between the two single nuts has at least one peripheral section pressed into an associated aperture in the second single nut. The advantages of this method and its possible forms are described above in connection with the discussion of the screw drive according to the invention.
In all the embodiments discussed above, it is firstly possible for the two single nuts to bear directly on each other in the direction of the axis of the spindle. Alternatively, it is also possible for the two single nuts to bear on each other in the direction of the axis of the spindle by means of the intermediate element.